Lithium cobalt oxide, method for manufacturing the same, and nonaqueous electrolyte secondary battery

ABSTRACT

Lithium cobalt oxide, which can provide a nonaqueous electrolyte secondary battery having an excellent initial capacity and an excellent capacity retention, and a method for manufacturing the same are provided. The lithium cobalt oxide has a tap density of at least 1.7 g/cm 3  and a pressed density of 3.5 to 4.0 g/cm 3 . A method for manufacturing the lithium cobalt oxide includes the step of selecting a lithium cobalt oxide (A) and a lithium cobalt oxide (B) so that a difference in the tap density between the lithium cobalt oxide (A) and the lithium cobalt oxide (B) is at least 0.2 g/cm 3 ; and mixing the lithium cobalt oxide (A) and the lithium cobalt oxide (B).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to lithium cobalt oxide, a method formanufacturing the same, and a nonaqueous electrolyte secondary batteryprovided with a cathode plate including the lithium cobalt oxide as acathode active material.

2. Description of the Related Art

In recent years, nonaqueous electrolyte secondary batteries, such aslithium ion secondary batteries, have become in practical use as powersources for small electronic equipment, e.g., laptop personal computers,cellular phones, and video cameras, in accordance with rapid advance ofportable and cordless electronic equipment intended for home use.

With respect to these lithium ion secondary batteries, since lithiumcobalt oxide is useful as a cathode active material for the lithium ionsecondary battery, research has been actively conducted on lithium-basedcomplex oxides, and many proposes have been made regarding compounds,e.g., lithium cobalt oxide, lithium nickel oxide, and lithium manganeseoxide, as cathode active materials until now.

Various proposes for improving the performances of those cathode activematerials have been made, and many technologies regarding the apparentdensity, the pressed density, and the like are disclosed as importantfactors.

For example, a cathode active material has been proposed, wherein thetap density of Li_(p)MO₂ containing granular composition prepared byfiring at least two types of starting materials having different averageparticle diameters is at least 2.65 g/cm³ (referred to, for example, thefront page of Japanese Unexamined Patent Application Publication No.2001-85009).

Another cathode active material for a nonaqueous electrolyte secondarybattery has been proposed, wherein with respect to a cathode activematerial used for a nonaqueous electrolyte secondary battery includinglithium cobalt oxide represented by a formula, LiCoO₂. Theabove-described lithium cobalt oxide is composed of spherical orellipsoidal secondary particles which have a Feret diameter ofprojection pattern of 0.1 to 4 μm on a SEM observation basis and inwhich many small crystalline primary particles have gathered while theprimary particles have an average particle diameter of 2 μm or less. Thetap density of the above-described lithium cobalt oxide is at least 2.2g/cm³ (referred to, for example, the front page of Japanese UnexaminedPatent Application Publication No. 2001-135313).

Another cathode active material for a nonaqueous electrolyte secondarybattery has been proposed, wherein the cathode active material includeslithium cobalt oxide composed of secondary particles in which many fineprimary particles of lithium cobalt oxide subsequently represented by ageneral formula, LiCoO₂, have gathered while the secondary particle hasmany fine gaps capable of being impregnated with an electrolyticsolution, and the lithium cobalt oxide has a tap density of at least 2.2g/cm³ (referred to, for example, the front page of Japanese UnexaminedPatent Application Publication No. 2001-155729).

However, any nonaqueous electrolyte secondary battery including theabove-described lithium cobalt oxide as a cathode active material doesnot simultaneously satisfy the discharge capacity and the quickcharge-discharge performance under present circumstances. Therefore,various attempts have been conducted. For example, it has been attemptedto change the particle diameter and the shape of particle in order toincrease the electrode density and, thereby, increase the batterycapacity and in order to improve the quick charge-discharge performance.However, satisfactory results have not yet been achieved.

SUMMARY OF THE INVENTION

The present invention has been made in consideration of theabove-described problems in known technologies. Accordingly, it is anobject of the present invention to provide lithium cobalt oxide havingexcellent powder properties, allowing the electrode density to becomehigh, and allowing a battery prepared by using the lithium cobalt oxideto have a large discharge capacity and excellent quick charge-dischargeperformance, to provide a method for manufacturing the same, and toprovide a nonaqueous electrolyte secondary battery including the lithiumcobalt oxide.

The inventors of the present invention found out that in the case wherelithium complex oxide particles were used as the cathode activematerial, when the particle properties were specified and, in addition,lithium complex oxide particles having different particle diameters wereblended, the most of the properties of the particles was able to beexerted. Consequently, the present invention has been completed.

The present invention relates to lithium cobalt oxide having a tapdensity of at least 1.7 g/cm³ and a pressed density of 3.5 to 4.0 g/cm³.

Preferably, the lithium cobalt oxide of the present invention is amixture of lithium cobalt oxide (A) composed of monodisperse primaryparticles and lithium cobalt oxide (B) composed of aggregated primaryparticles, and the mixture has a tap density of at least 1.7 g/cm³ and apressed density of 3.5 to 4.0 g/cm³.

The present invention relates to a method for manufacturing theabove-described lithium cobalt oxide, the method including the step ofmixing lithium cobalt oxide (A) having a tap density of 1.7 to 3.0 g/cm³and lithium cobalt oxide (B) having a tap density of 1.0 to 2.0 g/cm³,wherein these are combined in order that the difference in tap densityis at least 0.20 g/cm³ between the above-described lithium cobalt oxide(A) and the above-described lithium cobalt oxide (B).

Preferably, the above-described lithium cobalt oxide (A) and the lithiumcobalt oxide (B) are mixed at a ratio of (A):(B)=95:5 to 60:40 on aweight basis.

Preferably, the above-described lithium cobalt oxide (A) is composed ofmonodisperse primary particles, and the above-described lithium cobaltoxide (B) is composed of aggregated primary particles.

Preferably, the above-described lithium cobalt oxide (A) has an averageparticle diameter of 5 to 30 μm, and the above-described lithium cobaltoxide (B) has an average particle diameter of 0.1 to 10 μm.

Furthermore, the present invention relates to a nonaqueous electrolytesecondary battery provided with a cathode plate configured by the use ofthe above-described lithium cobalt oxide as a cathode active material.

The lithium cobalt oxide of the present invention can have a highpressed density and an appropriate tap density by mixing two differenttypes of lithium cobalt oxide, and when it is used as a cathode activematerial for a cathode plate, the effect of increasing the electrodedensity is exerted.

According to the manufacturing method of the present invention, theabove-described lithium cobalt oxide serving a useful function as acathode active material can readily be provided.

Furthermore, according to the present invention, a nonaqueouselectrolyte secondary battery which exhibits high degree of safety andhas excellent quick charge-discharge performance can be provided by theuse of the above-described lithium cobalt oxide as a cathode activematerial.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a SEM photograph (magnification 3,000 times) showing theparticle structure of the lithium cobalt oxide (A) composed of uniformlymonodisperse primary particles, in Manufacturing Example 1.

FIG. 2 is a SEM photograph (magnification 3,000 times) showing theparticle structure of the lithium cobalt oxide (B) composed ofaggregated primary particles, in Manufacturing Example 7.

FIG. 3 is a diagram showing the evaluation of safety of the secondarybatteries in which the lithium cobalt oxide in Example 2 and that inComparative Example 1 are used as respective cathode active materials.

FIG. 4 is a diagram showing the results of quick charge-discharge testsof secondary batteries in which the lithium cobalt oxide in Example 2and that in Comparative Example 1 are used as respective cathode activematerials.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Lithium cobalt oxide of the present invention has a tap density of atleast 1.7 g/cm³ and a pressed density of 3.5 to 4.0 g/cm³.

The above-described lithium cobalt oxide is composed of a mixture of atleast two compounds selected from the compounds represented by a generalformula (1), Li_(a)CoO₂ (in the formula, a represents a number withinthe range of 0.2≦a≦1.2), or a mixture of a compound represented by ageneral formula (1), Li_(a)CoO₂, and a compound represented by a generalformula (2), Li_(a)Co_(1-y)M_(y)O_(2-z) (in the formula, “M” representsat least one element selected from the group consisting of transitionmetal elements other than Co and elements having an atomic number of atleast 9, “a” represents a number within the range of 0.2≦a≦1.2, “y”represents a number within the range of 0<y≦0.4, and “z” represents anumber within the range of 0≦z≦1.0).

Specifically, other metal elements “M” may substitute a part ofLi_(a)CoO₂ or a part of Co in Li_(a)CoO₂. The metal element “M” is atleast one element selected from the group consisting of transition metalelements other than Co and elements having an atomic number of at least9, and is at least one element selected from, for example, Na, Mg, Al,Ca, Ti, V, Cr, Mn, Fe, Ni, Zn, Si, Ga, Zr, Nb, W, and Mo.

Alternatively, a surface of lithium cobalt oxide where other metalelements “M” have been substituted for a part of Li_(a)CoO₂ or a part ofCo in Li_(a)CoO₂ may be coated with a sulfate.

In general, a tap density indicates a natural packing property of amixed powder of coarse particles and fine particles without beingpressurized intentionally. A pressed density indicates a packingproperty of coarse particles and fine particles under pressure. Thepresent invention is based on a finding that lithium cobalt oxide havinga tap density and a pressed density each within a specific range isimportant when the lithium cobalt oxide is used as a cathode activematerial of a nonaqueous electrolyte secondary battery.

It is desirable that the tap density of the lithium cobalt oxide of thepresent invention is at least 1.7 g/cm³, preferably is 2.0 to 3.0 g/cm³.

It is desirable that the pressed density of the lithium cobalt oxide ofthe present invention is 3.5 to 4.0 g/cm³, preferably is 3.6 to 4.0g/cm³, and further preferably is 3.7 to 4.0 g/cm³.

The lithium cobalt oxide of the present invention has a tap density anda pressed density within the above-described ranges and, thereby, hasexcellent properties as the cathode active material.

A method for manufacturing the lithium cobalt oxide of the presentinvention will now be described.

In this manufacturing method, the lithium cobalt oxide of the presentinvention may be produced by dry mixing of at least two types of lithiumcobalt oxide having different tap densities.

Specifically, the method for manufacturing the lithium cobalt oxideaccording to the present invention includes the step of mixing lithiumcobalt oxide (A) having a tap density of 1.7 to 3.0 g/cm³ and lithiumcobalt oxide (B) having a tap density of 1.0 to 2.0 g/cm³, wherein theseare selected in order that the difference in tap density is at least0.20 g/cm³ between the above-described lithium cobalt oxide (A) and theabove-described lithium cobalt oxide (B).

Preferably, the above-described lithium cobalt oxide (A) and the lithiumcobalt oxide (B) are mixed at a ratio of (A):(B)=95:5 to 60:40 on aweight basis, and preferably at a ratio of 90:10 to 80:20.

The lithium cobalt oxide (A) to be used may have a tap density of 1.7 to3.0 g/cm³, and preferably of 2.0 to 3.0 g/cm³.

The lithium cobalt oxide (B) to be used may have a tap density of 1.0 to2.0 g/cm³, and preferably of 1.0 to 1.7 g/cm³.

Preferably, the lithium cobalt oxide (A) and (B) to be used havedifferent tap densities, and it is desirable that the difference in tapdensity between the above-described lithium cobalt oxide (A) and (B) isat least 0.20 g/cm³, and preferably is at least 0.30 g/cm³.

Preferably, the lithium cobalt oxide (A) is composed of monodisperseprimary particles. The monodisperse primary particles refer to particlesof a minimum size that are present independently of each other, andspecifically, this is checked by a scanning electron microscope (SEM)photograph observation. A powder in which monodisperse particlesconstitute at least 80 percent of the field of view provided by SEM maybe referred to as a monodisperse powder. FIG. 1 is a SEM photograph(magnification 3,000 times) showing the particle structure of thelithium cobalt oxide (A) composed of uniformly monodisperse primaryparticles, in Manufacturing Example 1.

It is desirable that the average particle diameter of theabove-described lithium cobalt oxide (A) is within the range of 5 to 30μm, and preferably is within the range of 10 to 20 μm. The lithiumcobalt oxide (A) is composed of coarse particles compared with thelithium cobalt oxide (B).

Preferably, the lithium cobalt oxide (B) is composed of secondaryparticles while a secondary particle is formed by aggregation of theprimary particles. The phrase “a secondary particle is formed byaggregation of primary particles” refers to a state in which particlesof minimum size are mutually attracted by van der Waals forces andsurface charge forces and, thereby, form the shape of a particle.Specifically, this is checked by a SEM photograph observation. A powderin which aggregated particles constitute at least 80 percent of thefield of view provided by SEM may be referred to as an aggregatedpowder. FIG. 2 is a SEM photograph (magnification 3,000 times) showingthe particle structure of the lithium cobalt oxide (B) composed ofaggregated primary particles, in Manufacturing Example 7.

It is desirable that the average particle diameter of theabove-described lithium cobalt oxide (B) is within the range of 0.1 to10 μm, and preferably is within the range of 2.0 to 8.0 μm.

The average particle diameter in the present invention indicates a valueat 50 percent (D₅₀) of the cumulative particle size distributionprovided by a laser scattering particle size distribution analyzer.

In the present invention, when the lithium cobalt oxide prepared bymixing the lithium cobalt oxide (B) composed of aggregated primaryparticles and the lithium cobalt oxide (A) composed of monodisperseprimary particles is used as a cathode active material of a nonaqueouselectrolyte secondary battery, excellent battery properties areexhibited. The reason for this is not clear, but it is believed that themixture of the above-described particles increases the packing densityon the cathode plate. In addition, the aggregated particles exhibitexcellent quick charge-discharge performance, while the monodisperseparticles ensure a high degree of safety.

Furthermore, in the manufacturing method of the present invention,preferably, the above-described lithium cobalt oxide (A) is a compoundrepresented by a general formula (1), Li_(a)CoO₂ (in the formula, “a”represents a number within the range of 0.2≦a≦1.2). Preferably, theabove-described lithium cobalt oxide (B) is a compound represented bythe above-described general formula (1) or a compound represented by ageneral formula (2), Li_(a)Co_(1-y)M_(y)O_(2-z) (in the formula, “M”represents at least one element selected from the group consisting oftransition metal elements other than Co and elements having an atomicnumber of at least 9, “a” represents a number within the range of0.2≦a≦1.2, “y” represents a number within the range of 0<y≦0.4, and “z”represents a number within the range of 0≦z≦1.0).

The lithium cobalt oxide of the present invention may be produced byuniformly mixing at least two types of lithium cobalt oxide havingdifferent tap densities and average particle diameters. A method usedfor uniform mixing is not specifically limited as long as the method isin practical use in the industry. Examples thereof include methodsthrough the use of, for example, rotary vessel type mixers, e.g.,horizontal cylinder type, V Type, and double-circular cone type, andfixed vessel type mixers, e.g., ribbon type, horizontal screw type,paddle type, vertical ribbon type, muller type, planetary motion type,static mixer, uniaxial rotor type, Henschel mixer, and Flo Jet mixer.

As the positive active material for a lithium secondary battery of cell,the lithium cobalt oxide compound described above is used. The positiveactive material is one of the stating materials for a positive electrodecompound of a lithium secondary cell. The positive electrode compound,which will be described later, is a mixture formed of the positiveactive material, a conductive agent, a binder, filler whenevernecessary, and the like. Since the positive active material of thelithium secondary cell, according to the present invention, is formed ofthe lithium cobalt compound oxide described above, kneading with theother starting materials can be easily performed when the positiveelectrode compound is prepared. In addition, coating of a positiveelectrode collector with the positive electrode compound thus obtainedcan also be easily performed.

The lithium secondary cell of the present invention uses the lithiumcobalt oxide compound as a positive active material and comprises apositive electrode, a negative electrode, separators, and a non-aqueouselectrolyte containing a lithium salt. The positive electrode is formed,for example, by applying a positive electrode compound onto a positiveelectrode collector, followed by drying, and the positive electrodecompound is composed of a positive active material, a conductive agent,a binder, and filler whenever necessary, and the like.

A material for the positive electrode collector is not particularlylimited as long it is inactive in an assembled cell in view of thechemical reaction. For example, the material may be mentioned stainlesssteel, nickel, aluminum, titanium, baked carbon, and aluminum orstainless steel surface-treated with carbon, nickel, titanium, orsilver.

As the conductive agent, for example, conductive materials may includematerials such as graphite including natural graphite and manmadegraphite, carbon black, acetylene black, carbon fiber, carbon nanotube,and metal such as powdered nickel. As the natural graphite, for example,scaly graphite, flake graphite, and earthy graphite may be mentioned.The conductive agents as mentioned above may be used alone or incombination. The content of the conductive agent in the positiveelectrode compound is 1 to 50 percent by weight and preferably 2 to 30percent by weight.

As the binder, for example, the following compounds may be usedpolysaccharides, thermoplastic resins, and polymers having elasticity,such as poly(vinylidene fluoride), poly(vinyl chloride),carboxylmethylcellulose, hydroxylpropylcellulose, recycled cellulose,diacetylcellulose, poly(vinyl pyrrolidone),ethylene-propylene-diene-terpolymer (EPDM), sulfonated EPDM,styrene-butadiene rubber, fluorinated rubber, and polyethylene oxide.The binders as mentioned above may be used alone or in combination. Thecontent of the binder in the positive electrode compound is 2 to 30percent by weight and preferably 5 to 15 percent by weight.

The filler of the positive electrode compound has a function ofsuppressing the volume expansion or the like of the positive electrodeand is used whenever it is necessary. As the filler, any fiber materialsmay be used as long they are inactive in an assembled cell in view ofthe chemical reaction. For example, fibers made of olefinic polymerssuch as polypropylene and polyethylene, glass fibers, and carbon fibersmay be used. The content of the filler is not particularly limited andis preferably 0 to 30 percent by weight of the positive electrodecompound.

The negative electrode is formed by applying a negative electrodematerial onto a negative electrode collector, followed by drying. As thenegative electrode collector, any material may be used as long as beinginactive in an assembled cell in view of the chemical reaction. Forexample, there may be mentioned stainless steel, nickel, copper,titanium, aluminum, baked carbon, copper or stainless steelsurface-treated with carbon, nickel, titanium, or silver, andaluminum-cadmium alloy.

The negative electrode material is not particularly limited, and forexample, may include carbonaceous materials, metal composite oxides,metal lithium, and lithium alloys. As the carbonaceous material, forexample, hard-graphitized carbon materials and graphite-based carbonmaterials may be mentioned. As the metal composite oxide, for example,there may be mentioned a compound represented SnM¹ _(1-x)M² _(y)O_(z)(where, “M¹” is at least one element selected from the group consistingof Mn, Fe, Pb, and Ge; “M²” is at least one element selected from thegroup consisting of Al, B, P, Si, elements of group I, II, and III ofthe periodic table, and halogen atoms; and 0<x≦1, 1≦y≦3, and 1≦z≦8 aresatisfied).

As the separator, an insulating thin film having a high iontransmittance and a predetermined mechanical strength is used. Sheetsand nonwoven cloths may be used which are made of glass fibers or anolefinic polymer, such as polyethylene or polypropylene, havingorganic-solvent resistance and hydrophobic properties. The pore diameterof the separator is not particularly limited as long as it iseffectively used for a general cell application and is, for example,0.01 to 10 μm. The thickness of the separator may be in the range usedfor a general cell application and is, for example, 5 to 300 μm. Inaddition, in the case in which a solid electrolyte such as a polymer isused as described later, the solid electrolyte may also be used as theseparator. In addition, in order to improve charge and dischargeproperties, a compound such as pyridine, triethyl phosphite, ortriethanolamine may be added to the electrolyte.

The non-aqueous electrolyte containing a lithium salt is a mixture of anon-aqueous electrolyte and a lithium salt. As the non-aqueouselectrolyte, a non-aqueous electrolyte or an organic solid electrolyteis used. As the non-aqueous electrolyte, for example, there may bementioned aprotic organic solvents such as N-methyl-2-pyrrolidinone,propylene carbonate, ethylene carbonate, butylene carbonate, dimethylcarbonate, diethyl carbonate, (-butyrolactone, 1,2-dimethoxyethane,tetrahydrofuran, 2-methyl tetrahydrofuran, dimethyl sulfoxide,1,3-dioxolane, formamide, dimethyl formamide, dioxolane, acetonitrile,nitromethane, methyl formate, methyl acetate, a phosphoric acidtriester, trimethoxymethane, a dioxolane derivative, sulfolane,3-methyl-2-oxazolidinone, a propylene carbonate derivative, atetrahydrofuran derivative, diethyl ether, and 1,3-propanesultone. Thosecompounds mentioned above may be used alone or in combination.

As the organic solid electrolyte, for example, a polyethylenederivative, a polymer including the same, a propylene oxide derivative,a polymer including the same, and a phosphate polymer may be mentioned.As the lithium salt, a material dissolved in the non-aqueous electrolytedescribed above is used, and for example, LiClO₄, LiBF₄, LiPF₆,LiCF₃SO₃, LiCF₃CO₂, LiAsF₆, LiSbF₆, LiB₁₀Cl₁₀, LiAlCl₄, chloroboronlithium, a lithium lower aliphatic carboxylate, and lithiumtetraphenylborate may be used alone or in combination.

The shape of the lithium secondary cell of the present invention may bea button, sheet, cylinder, rectangle, or the like. The application ofthe secondary cell of the present invention is not particularly limitedand may be applied to electronic apparatuses, such as notebook personalcomputers, laptop personal computers, pocket type word processors,mobile phones, cordless phone handsets, portable CD players, and radios,and consumer electronic apparatuses for automobiles, electric vehicles,and game machines. In addition, the lithium secondary cell iscategorized as a non-aqueous electrolyte secondary cell.

The present invention provides portable electronic apparatusesincorporating the non-aqueous electrolyte secondary cell describedabove. As the portable electronic apparatuses, for example, notebookpersonal computers, pocket type word processors, mobile phones, cordlessphone handsets, portable CD players, radios, and game machines may beused.

EXAMPLES

The present invention will be described below in further detail withreference to the examples.

In the examples, a cathode active material and a nonaqueous electrolytesecondary battery of the present invention will be described.

(1) Method for Measuring Tap Density

A graduated cylinder is dried completely, and the weight of the emptygraduated cylinder is measured. Approximately 70 g of sample is weighedon weighing paper. The sample is transferred into the graduated cylinderby using a funnel. The graduated cylinder is set in an automated TDanalyzer (Dual Autotap produced by YUASA-IONICS COMPANY, LIMITED), thenumber of taps is set at 500, and tapping is the performed. The tap israised to 3.2 mm above the sample, and the tapping pace is 200 times/min(in accordance with ASTM: B527-93, 85). After the 500 taps arecompleted, the scale at the level of the sample surface is read, theweight of the graduated cylinder including the sample is measured and,thereby, a tap density is calculated.

(2) Method for Measuring Pressed Density

A sample is put in a mold of 15 mm in diameter, pressing (Hand Press;Type WPN-10, produced by ToYo Shoko) of 1.96×10⁸ Pa (2 ton/cm²) isperformed for 1 minute, so that a pellet is prepared. Subsequently, theweight and the volume of the resulting pellet are measured, the densityof the pellet is calculated, and this is taken as a pressed density.

Manufacturing Example 1

Lithium carbonate and cobalt oxide were weighed in order so that theLi/Co atomic ratio became 1.02, and they were adequately mixed in amortar, so that a uniform mixture was prepared. The resulting mixturewas filled in an alumina crucible, and was put in an electric furnace.The temperature was raised in the atmosphere, and a firing treatment wasperformed by keeping the mixture at a temperature of 700° C. to 1,000°C. for 10 hours. The resulting fired material was cooled in the air and,thereafter, grinding and classification were performed, so that lithiumcobalt oxide (LiCoO2) having an average particle diameter of 15.5 μm, atap density of 2.80 g/cm³, and a pressed density of 3.45 g/cm³ wasprepared.

The resulting lithium cobalt oxide was a lithium cobalt oxide (A-1)composed of uniformly monodisperse primary particles.

Manufacturing Example 2

As described in Manufacturing Example 1, lithium carbonate and cobaltoxide were mixed in order so that the Li/Co atomic ratio became 1.04and, thereby, a uniform mixture was prepared. A firing treatment wasperformed at 1,000° C. to 1,050° C. for 10 hours, so that lithium cobaltoxide (LiCoO₂) having an average particle diameter of 12.3 μm, a tapdensity of 2.50 g/cm³, and a pressed density of 3.48 g/cm³ was prepared.

The SEM image of the resulting lithium cobalt oxide indicated that alithium cobalt oxide (A-2) was composed of uniformly monodisperseprimary particles.

Manufacturing Example 3

As described in Manufacturing Example 1, lithium carbonate and cobaltoxide were mixed in order so that the Li/Co atomic ratio became 1.02and, thereby, a uniform mixture was prepared. Firing was performed at1,000° C. to 1,050° C. for 10 hours, so that lithium cobalt oxide havingan average particle diameter of 7.8 μm, a tap density of 1.90 g/cm³, anda pressed density of 3.41 g/cm³ was prepared. The resulting lithiumcobalt oxide was a lithium cobalt oxide (A-3) composed of uniformlymonodisperse primary particles.

Manufacturing Example 4

As described in Manufacturing Example 1, lithium carbonate and cobaltoxide were mixed in order so that the Li/Co atomic ratio became 1.00and, thereby, a uniform mixture was prepared. A firing treatment wasperformed at 900° C. to 1,000° C. for 10 hours, so that lithium cobaltoxide (LiCoO₂) having an average particle diameter of 7.4 μm, a tapdensity of 1.80 g/cm³, and a pressed density of 3.20 g/cm³ was prepared.

The resulting lithium cobalt oxide was a lithium cobalt oxide (B-1)composed of aggregated primary particles.

Manufacturing Example 5

As described in Manufacturing Example 1, lithium carbonate and cobaltoxide were mixed in order so that the Li/Co atomic ratio became 1.00and, thereby, a uniform mixture was prepared. A firing treatment wasperformed at 900° C. to 1,000° C. for 10 hours, so that lithium cobaltoxide (LiCoO₂) having an average particle diameter of 5.2 μm, a tapdensity of 1.50 g/cm³, and a pressed density of 3.15 g/cm³ was prepared.

The resulting lithium cobalt oxide was a lithium cobalt oxide (B-2)composed of aggregated primary particles.

Manufacturing Example 6

As described in Manufacturing Example 1, lithium carbonate and cobaltoxide were mixed in order so that the Li/Co atomic ratio became 1.00and, thereby, a uniform mixture was prepared. A firing treatment wasperformed at 800° C. to 900° C. for 10 hours, so that lithium cobaltoxide (LiCoO₂) having an average particle diameter of 3.2 μm, a tapdensity of 1.20 g/cm³, and a pressed density of 3.21 g/cm³ was prepared.

The SEM image of the resulting lithium cobalt oxide indicated that alithium cobalt oxide (B-3) was composed of aggregated primary particles.

Manufacturing Example 7

In a manner similar to that in Manufacturing Example 1,LiCo_(0.98)Al_(0.02)O₂ was synthesized, in which 2 mole percent of Alwas added relative to Co. The firing method was similar to that inManufacturing Example 1. Mixing was performed in a mortar in order that2 mole percent of Al(OH)₃ was included relative to Co and, thereafter,firing was performed at 800° C. to 900° C., so that lithium cobalt oxide(LiCo_(0.98)Al_(0.02)O₂) was prepared.

The resulting lithium cobalt oxide had an average particle diameter of2.8 μm, a tap density of 1.18 g/cm³, and a pressed density of 3.19g/cm³.

The SEM image of the resulting lithium cobalt oxide indicated that alithium cobalt oxide (B-4) was composed of aggregated primary particles.

FIG. 1 is a SEM photograph (magnification 3,000 times) showing theparticle structure of the lithium cobalt oxide (A) composed of uniformlymonodisperse primary particles, in Manufacturing Example 1.

FIG. 2 is a SEM photograph (magnification 3,000 times) showing theparticle structure of lithium cobalt oxide (B) composed of aggregatedprimary particles, in Manufacturing Example 7.

The lithium cobalt oxide (A) and (B) prepared in the above-describedManufacturing Examples 1 to 7 are collectively shown in Table 1.

TABLE 1 Average particle Tap Pressed diameter density density Lithiumcobalt oxide (μm) (g/cm³) (g/cm³) Manufacturing A-1 Monodispersion 15.502.80 3.45 Example 1 Manufacturing A-2 Monodispersion 12.30 2.50 3.48Example 2 Manufacturing A-3 Monodispersion 7.80 1.90 3.41 Example 3Manufacturing B-1 Aggregation 7.40 1.80 3.20 Example 4 Manufacturing B-2Aggregation 5.20 1.50 3.15 Example 5 Manufacturing B-3 Aggregation 3.201.20 3.21 Example 6 Manufacturing B-4 Aggregation 2.80 1.18 3.19 Example7

(Note) In Table 1, A represents lithium cobalt oxide (A), and Brepresents lithium cobalt oxide (B).

It is desirable that the pressed density of the lithium cobalt oxide (A)in the present invention is 3.3 to 3.7 g/cm³, and preferably is 3.5 to3.7 g/cm³. It is desirable that the pressed density of the lithiumcobalt oxide (B) in the present invention is 3.1 to 3.5 g/cm³, andpreferably is 3.1 to 3.3 g/cm³.

In the present invention, it is desirable that the difference in presseddensity between the lithium cobalt oxide (A) and the lithium cobaltoxide (B) is at least 0.2 g/cm³, and preferably is 0.8 to 1.5 g/cm³.

Example 1

Lithium cobalt oxide was prepared by uniformly mixing 95 parts by weightof lithium cobalt oxide (A-1) having an average particle diameter of15.5 μm and a tap density of 2.80 g/cm³, prepared in ManufacturingExample 1, and 5 parts by weight of lithium cobalt oxide (B-3) having anaverage particle diameter of 3.2 μm and a tap density of 1.20 g/cm³,prepared in Manufacturing Example 6, with a small ribbon mixer. Theresulting lithium cobalt oxide had an average particle diameter of 15.0μm, a tap density of 2.75 g/cm³, and a pressed density of 3.65 g/cm³.

Example 2

Lithium cobalt oxide was prepared by uniformly mixing 70 parts by weightof lithium cobalt oxide (A-1) having an average particle diameter of15.5 μm and a tap density of 2.80 g/cm³, prepared in ManufacturingExample 1, and 30 parts by weight of lithium cobalt oxide (B-3) havingan average particle diameter of 3.2 μm and a tap density of 1.20 g/cm³,prepared in Manufacturing Example 6. The resulting lithium cobalt oxidehad an average particle diameter of 11.9 μm, a tap density of 2.40g/cm³, and a pressed density of 3.92 g/cm³.

Example 3

Lithium cobalt oxide was prepared by uniformly mixing 70 parts by weightof lithium cobalt oxide (A-1) having an average particle diameter of15.5 μm and a tap density of 2.80 g/cm³, prepared in ManufacturingExample 1, and 30 parts by weight of lithium cobalt oxide (B-2) havingan average particle diameter of 5.2 μm and a tap density of 1.50 g/cm³,prepared in Manufacturing Example 5. The resulting lithium cobalt oxidehad an average particle diameter of 12.8 μm, a tap density of 2.53g/cm³, and a pressed density of 3.82 g/cm³.

Example 4

Lithium cobalt oxide was prepared by uniformly mixing 80 parts by weightof lithium cobalt oxide (A-2) having an average particle diameter of12.3 μm and a tap density of 2.50 g/cm³, prepared in ManufacturingExample 2, and 20 parts by weight of lithium cobalt oxide (B-2) havingan average particle diameter of 5.2 μm and a tap density of 1.50 g/cm³,prepared in Manufacturing Example 5. The resulting lithium cobalt oxidehad an average particle diameter of 10.5 μm, a tap density of 2.40g/cm³, and a pressed density of 3.75 g/cm³.

Example 5

Lithium cobalt oxide was prepared by uniformly mixing 60 parts by weightof lithium cobalt oxide (A-2) having an average particle diameter of12.3 μm and a tap density of 2.50 g/cm³, that was prepared inManufacturing Example 2, and 40 parts by weight of lithium cobalt oxide(B-1) having an average particle diameter of 7.4 μm and a tap density of1.80 g/cm³, prepared in Manufacturing Example 4. The resulting lithiumcobalt oxide had an average particle diameter of 10.1 μm, a tap densityof 2.35 g/cm³, and a pressed density of 3.65 g/cm³.

Example 6

Lithium cobalt oxide was prepared by uniformly mixing 85 parts by weightof lithium cobalt oxide (A-3) having an average particle diameter of 7.8μm and a tap density of 1.90 g/cm³, prepared in Manufacturing Example 3,and 15 parts by weight of lithium cobalt oxide (B-3) having an averageparticle diameter of 3.2 μm and a tap density of 1.20 g/cm³, prepared inManufacturing Example 6. The resulting lithium cobalt oxide had anaverage particle diameter of 7.0 μm, a tap density of 1.83 g/cm³, and apressed density of 3.55 g/cm³.

Example 7

Lithium cobalt oxide was prepared by uniformly mixing 60 parts by weightof lithium cobalt oxide (A-1) having an average particle diameter of15.5 μm and a tap density of 2.80 g/cm³, prepared in ManufacturingExample 1, and 40 parts by weight of lithium cobalt oxide (B-3) havingan average particle diameter of 3.2 μm and a tap density of 1.20 g/cm³,prepared in Manufacturing Example 6. The resulting lithium cobalt oxidehad an average particle diameter of 7.8 μm, a tap density of 1.88 g/cm³,and a pressed density of 3.50 g/cm³.

Example 8

Lithium cobalt oxide was prepared by uniformly mixing 70 parts by weightof lithium cobalt oxide (A-1) having an average particle diameter of15.5 μm and a tap density of 2.80 g/cm³, prepared in ManufacturingExample 1, and 30 parts by weight of Al-containing lithium cobalt oxide(LiCo_(0.98)Al_(0.02)O₂) (B-4) having an average particle diameter of2.8 μm and a tap density of 1.18 g/cm³, prepared in ManufacturingExample 7. The resulting lithium cobalt oxide had an average particlediameter of 7.7 μm, a tap density of 2.38 g/cm³, and a pressed densityof 3.89 g/cm³.

Example 9

Lithium cobalt oxide was prepared by uniformly mixing 90 parts by weightof lithium cobalt oxide (A-1) having an average particle diameter of15.5 μm and a tap density of 2.80 g/cm³, prepared in ManufacturingExample 1, and 10 parts by weight of lithium cobalt oxide (B-3) havingan average particle diameter of 3.2 μm and a tap density of 1.20 g/cm³,prepared in Manufacturing Example 6. The resulting lithium cobalt oxidehad an average particle diameter of 13.8 μm, a tap density of 2.65g/cm³, and a pressed density of 3.72 g/cm³.

Example 10

Lithium cobalt oxide was prepared by uniformly mixing 90 parts by weightof lithium cobalt oxide (A-1) having an average particle diameter of15.5 μm and a tap density of 2.80 g/cm³, prepared in ManufacturingExample 1, and 10 parts by weight of lithium cobalt oxide (B-1) havingan average particle diameter of 7.4 μm and a tap density of 1.80 g/cm³,prepared in Manufacturing Example 4. The resulting lithium cobalt oxidehad an average particle diameter of 14.8 μm, a tap density of 2.70g/cm³, and a pressed density of 3.60 g/cm³.

Example 11

Lithium cobalt oxide was prepared by uniformly mixing 80 parts by weightof lithium cobalt oxide (A-1) having an average particle diameter of15.5 μm and a tap density of 2.80 g/cm³, prepared in ManufacturingExample 1, and 20 parts by weight of lithium cobalt oxide (B-3) havingan average particle diameter of 3.2 μm and a tap density of 1.20 g/cm³,prepared in Manufacturing Example 6. The resulting lithium cobalt oxidehad an average particle diameter of 13.2 μm, a tap density of 2.58g/cm³, and a pressed density of 3.74 g/cm³.

Example 12

Lithium cobalt oxide was prepared by uniformly mixing 80 parts by weightof lithium cobalt oxide (A-1) having an average particle diameter of15.5 μm and a tap density of 2.80 g/cm³, prepared in ManufacturingExample 1, and 20 parts by weight of lithium cobalt oxide (B-1) havingan average particle diameter of 7.4 μm and a tap density of 1.80 g/cm³,prepared in Manufacturing Example 4. The resulting lithium cobalt oxidehad an average particle diameter of 13.8 μm, a tap density of 2.62g/cm³, and a pressed density of 3.58 g/cm³.

The lithium cobalt oxide prepared in the above-described ManufacturingExamples 1 to 12 by mixing the lithium cobalt oxide (A) and (B) arecollectively shown in Tables 2 and 3.

TABLE 2 (A) (B) Tap Mixture Average Tap Tap density Mixing tap particledensity density difference ratio density diameter g/cm³ g/cm³ g/cm³ A:Bg/cm³ μm Example 1 2.80 1.20 1.60 95:5  2.75 15.0 A-1 B-3 Example 2 2.801.20 1.60 70:30 2.40 11.9 A-1 B-3 Example 3 2.80 1.50 1.30 70:30 2.5312.8 A-1 B-2 Example 4 2.50 1.50 1.00 80:20 2.40 10.5 A-2 B-2 Example 52.50 1.80 0.70 60:40 2.35 10.1 A-2 B-1 Example 6 1.90 1.20 0.70 85:151.83 7.0 A-3 B-3 Example 7 2.80 1.20 1.60 60:40 1.88 7.8 A-1 B-3 Example8 2.80 1.18 1.62 70:30 2.38 7.7 A-1 B-4 Example 9 2.80 1.20 1.60 90:102.65 13.8 A-1 B-3 Example 2.80 1.80 1.00 90:10 2.70 14.8 10 A-1 B-1Example 2.80 1.20 1.60 80:20 2.58 13.2 11 A-1 B-3 Example 2.80 1.80 1.0080:20 2.62 13.8 12 A-1 B-1

TABLE 3 (A) (B) Pressed Mixture Average Pressed Pressed density Mixingpressed particle density density difference ratio density diameter g/cm³g/cm³ g/cm³ A:B g/cm³ μm Example 1 3.45 3.21 0.24 95:5  3.65 15.0 A-1B-3 Example 2 3.45 3.21 0.24 70:30 3.95 11.9 A-1 B-3 Example 3 3.45 3.150.30 70:30 3.82 12.8 A-1 B-2 Example 4 3.48 3.15 0.33 80:20 3.75 10.5A-2 B-2 Example 5 3.48 3.20 0.28 60:40 3.65 10.1 A-2 B-1 Example 6 3.413.21 0.20 85:15 3.55 7.0 A-3 B-3 Example 7 3.45 3.21 0.24 60:40 3.50 7.8A-1 B-3 Example 8 3.45 3.19 0.26 70:30 3.89 7.7 A-1 B-4 Example 9 3.453.21 0.24 90:10 3.72 13.8 A-1 B-3 Example 3.45 3.20 0.25 90:10 3.60 14.810 A-1 B-1 Example 3.45 3.21 0.24 80:20 3.74 13.2 11 A-1 B-3 Example3.45 3.20 0.25 80:20 3.58 13.8 12 A-1 B-1

(Note) In Tables 2 and 3, A represents lithium cobalt oxide (A), and Brepresents lithium cobalt oxide (B).

Comparative Example 1

Lithium cobalt oxide (LiCoO₂) having an average particle diameter of12.3 μm, a tap density of 2.50 g/cm³, and a pressed density of 3.48g/cm³ is shown as a comparative example.

The SEM image of this lithium cobalt oxide indicates that the lithiumcobalt oxide (A) is composed of uniformly monodisperse primaryparticles.

Comparative Examples of lithium cobalt oxide prepared by changing themixing of the lithium cobalt oxide (A) and the lithium cobalt oxide (B)of the present invention will be described below.

Manufacturing Example 8 Comparative Manufacturing Example

Lithium carbonate and cobalt oxide were mixed in order so that the Li/Coatomic ratio became 1.00 and, thereby, a uniform mixture was prepared. Afiring treatment was performed by keeping at 1,000° C. to 1,050° C. for10 hours. The resulting fired material was ground and classified in theair, so that lithium cobalt oxide (LiCoO₂) having an average particlediameter of 4.5 μm, a tap density of 1.60 g/cm³, and a pressed densityof 3.25 g/cm³ was prepared.

The resulting lithium cobalt oxide was a lithium cobalt oxide (C-1)composed of uniformly monodisperse primary particles.

Manufacturing Example 9 Comparative Manufacturing Example

Lithium carbonate and cobalt oxide were mixed in order so that the Li/Coatomic ratio became 1.00 and, thereby, a uniform mixture was prepared. Afiring treatment was performed by keeping at 800° C. to 850° C. for 10hours. The resulting fired material was ground and classified in theair, so that lithium cobalt oxide (LiCoO₂) having an average particlediameter of 11.0 μm, a tap density of 2.20 g/cm³, and a pressed densityof 3.30 g/cm³ was prepared.

The resulting lithium cobalt oxide was a lithium cobalt oxide (C-2)composed of aggregated primary particles.

Manufacturing Example 10 Comparative Manufacturing Example

Lithium carbonate and cobalt oxide were mixed in order so that the Li/Coatomic ratio became 1.00 and, thereby, a uniform mixture was prepared. Afiring treatment was performed by keeping at 1,000° C. to 1,050° C. for10 hours. The resulting fired material was ground and classified in theair, so that lithium cobalt oxide (LiCoO₂) having an average particlediameter of 5.0 μm, a tap density of 1.32 g/cm³, and a pressed densityof 3.12 g/cm³ was prepared.

The resulting lithium cobalt oxide was a lithium cobalt oxide (C-3)composed of uniformly monodisperse primary particles.

Comparative Example 2

Lithium cobalt oxide was prepared by uniformly mixing 80 parts by weightof lithium cobalt oxide (C-1) prepared in Manufacturing Example 8 and 20parts by weight of lithium cobalt oxide (B-3) prepared in ManufacturingExample 6 with a small ribbon mixer.

The resulting lithium cobalt oxide had an average particle diameter of3.5 μm, a tap density of 1.32 g/cm³, and a pressed density of 3.22g/cm³.

Comparative Example 3

Lithium cobalt oxide was prepared by uniformly mixing 80 parts by weightof lithium cobalt oxide (C-2) prepared in Manufacturing Example 9 and 20parts by weight of lithium cobalt oxide (B-3) prepared in ManufacturingExample 6 with a small ribbon mixer.

The resulting lithium cobalt oxide had an average particle diameter of11.2 μm, a tap density of 2.15 g/cm³, and a pressed density of 3.45g/cm³.

Comparative Example 4

Lithium cobalt oxide was prepared by uniformly mixing 80 parts by weightof lithium cobalt oxide (A-1) prepared in Manufacturing Example 1 and 20parts by weight of lithium cobalt oxide (C-2) prepared in ManufacturingExample 9 with a small ribbon mixer.

The resulting lithium cobalt oxide had an average particle diameter of11.2 μm, a tap density of 2.56 g/cm³, and a pressed density of 3.37g/cm³.

Comparative Example 5

Lithium cobalt oxide was prepared by uniformly mixing 80 parts by weightof lithium cobalt oxide (A-1) prepared in Manufacturing Example 1 and 20parts by weight of lithium cobalt oxide (C-3) prepared in ManufacturingExample 10 with a small ribbon mixer.

The resulting lithium cobalt oxide had an average particle diameter of13.4 μm, a tap density of 2.62 g/cm³, and a pressed density of 3.40g/cm³.

Comparative Example 6

Lithium cobalt oxide was prepared by uniformly mixing 50 parts by weightof lithium cobalt oxide (A-1) prepared in Manufacturing Example 1, and50 parts by weight of lithium cobalt oxide (B-3) prepared inManufacturing Example 6 with a small ribbon mixer.

The resulting lithium cobalt oxide had an average particle diameter of9.5 μm, a tap density of 1.71 g/cm³, and a pressed density of 3.42g/cm³.

TABLE 4 Average particle Tap Pressed diameter density density Lithiumcobalt oxide (μm) (g/cm³) (g/cm³) Manufacturing A-2 Monodispersion 12.32.50 3.48 Example 2 Manufacturing C-1 Monodispersion 4.5 1.60 3.25Example 8 Manufacturing C-2 Aggregation 11.0 2.20 3.30 Example 9Manufacturing C-3 Monodispersion 5.0 1.32 3.12 Example 10

TABLE 5 Average (A) (B) Tap Mixture particle Tap Tap density Mixing tapdiam- density density difference ratio density eter g/cm³ g/cm³ g/cm³A:B g/cm³ μm Comparative 2.50 — — 100 2.50 12.3 Example 1 A-2Comparative 1.60 1.20 0.40 80:20 1.32 3.5 Example 2 C-1 B-3 Comparative2.20 1.20 1.00 80:20 2.15 11.2 Example 3 C-2 B-3 Comparative 2.80 2.200.60 80:20 2.56 11.2 Example 4 A-1 C-2 Comparative 2.80 1.32 1.48 80:202.62 13.4 Example 5 A-1 C-3 Comparative 2.80 1.20 1.60 50:50 1.71 9.5Example 6 A-1 B-3

TABLE 6 Average (A) (B) Pressed Mixture particle Pressed Pressed densityMixing pressed diam- density density difference ratio density eter g/cm³g/cm³ g/cm³ A:B g/cm³ μm Comparative 3.48 — — 100 3.48 12.3 Example 1A-2 Comparative 3.25 3.21 0.04 80:20 3.22 3.5 Example 2 C-1 B-3Comparative 3.30 3.21 0.09 80:20 3.45 11.2 Example 3 C-2 B-3 Comparative3.45 3.30 0.15 80:20 3.37 11.2 Example 4 A-1 C-2 Comparative 3.45 3.120.33 80:20 3.40 13.4 Example 5 A-1 C-3 Comparative 3.45 3.21 0.24 50:503.42 9.5 Example 6 A-1 B-3

The results of the evaluation of safety and the quick charge-dischargetests of secondary batteries in which the lithium cobalt oxide of thepresent invention is used as the cathode active material will bedescribed below.

(Evaluation of Safety)

Each of the lithium cobalt oxides in Example 2 and the lithium cobaltoxide (A) of Comparative Example 1 was used as a cathode activematerial. The cathode active material was applied on an aluminum foil.The resulting cathode plate was used, and a lithium ion secondarybattery was prepared through the use of various elements, e.g., aseparator, an anode, a cathode, a current collector, mounting fittings,external terminals, and an electrolytic solution. Among them, metallithium was used as the anode, and a solution prepared by dissolving 1mol of LiPF₆ into 1 liter of mixed solution of ethylene carbonate (EC)and methyl ethyl carbonate (MEC) in a 1:1 mixing ratio was used as theelectrolytic solution. After a battery was prepared, charging wasperformed at 4.3 V versus Li/Li⁺, and adequate cleaning was performedwith acetone, followed by drying. The resulting electrode washermetically sealed in a vessel together with the mixed solution ofethylene carbonate (EC) and methyl ethyl carbonate (MEC) in a 1:1 mixingratio, which was used for the electrolytic solution. Subsequently, thethermal stability test was performed based on a DSC measurement. Theresults thereof are shown in FIG. 3.

As is clear from the results shown in FIG. 3, the first exothermic peak(lower temperature side is approximately 180° C.) of Example 2 issmaller than the first exothermic peak (lower temperature side isapproximately 180° C.) of Comparative Example 1 and, therefore, theheating value is small. Consequently, it is clear that the activematerial of Example 2 exhibits a higher degree of safety compared withthe active material of Comparative Example 1. In general, fine particleshave a large specific surface area and high reactivity and, therefore,exhibit low degree of safety. However, since coarse particles (lithiumcobalt oxide (A)) coexist in the lithium cobalt oxide of Example 2, itis estimated that the high degree of safety of the coarse particlescanceled the low degree of safety of the fine particles (lithium cobaltoxide (B)), and this effect contributed to the above-described result.

(Quick Charge-Discharge Test)

Each of the lithium cobalt oxide in Example 2 and the lithium cobaltoxide (A) of Comparative Example 1 was used as a cathode activematerial. The cathode active material was applied on an aluminum foil.The resulting cathode plate was used, and a lithium ion secondarybattery was prepared through the use of various elements, e.g., aseparator, an anode, a cathode, a current collector, mounting fittings,external terminals, and an electrolytic solution. Among them, metallithium was used as the anode, and a solution prepared by dissolving 1mol of LiPF₆ into 1 liter of mixed solution of ethylene carbonate (EC)and methyl ethyl carbonate (MEC) in a 1:1 mixing ratio was used as theelectrolytic solution.

A constant-current charge-discharge test was performed at 2.7 V to 4.3 V(vs. Li/Li⁺), and the resulting charge-discharge curve is shown in FIG.4. At that time, the current value was increased as 0.2 C→0.5 C→1.0C→2.0 C (1.0 C→discharge over 1 hour, 2.0 C→discharge over 0.5 hours),and the quick charge-discharge performance was tested. The cathode andthe anode were metal Li, the electrolytic solution was 1 MLiPF₆/(EC+MEC), the charging system was CCCV (0.5 C, 5 H), and thescanning potentials were 2.7 V and 4.3 V.

As is clear from the results shown in FIG. 4, the capacity of Example 2,which can be taken out, is larger than that of Comparative Example 1.The reason is estimated that the fine particles (lithium cobalt oxide(B)) contained in the lithium cobalt oxide of Example 2 had outstandingquick charge-discharge performance and, thereby, excellent propertieswere exhibited.

Large particles contribute to the high degree of safety. Small particlesenter gaps between large particles and, thereby, high quickcharge-discharge performance can be achieved due to an increase in theconductivity between particles. However, if the pressed density is toohigh (at least 4.0), the electrode density of the resulting electrode isexcessively increased, the electrode is not adequately impregnated withthe electrolytic solution and, thereby, the quick charge-dischargeperformance improperly deteriorates. If the pressed density and the tapdensity are improper values, satisfactory electrode density cannot beachieved.

The pressed density of the lithium cobalt oxide in the present inventionwill be described below.

The lithium cobalt oxide of the present invention is a mixture of thelithium cobalt oxide (A) composed of monodisperse primary particles andthe lithium cobalt oxide (B) composed of aggregated primary particles,and the mixture has a tap density of at least 1.7 g/cm³ and a presseddensity of 3.5 to 4.0 g/cm³.

In a preferred embodiment of the above-described lithium cobalt oxide ofthe present invention, preferably, the lithium cobalt oxide is a mixtureof the lithium cobalt oxide (A) having a tap density of 1.7 to 3.0 g/cm³and a pressed density of 3.3 to 3.7 g/cm³ and the lithium cobalt oxide(B) having a tap density of 1.0 to 2.0 g/cm³ and a pressed density of3.1 to 3.5 g/cm³, and the difference in tap density between the lithiumcobalt oxide (A) and the lithium cobalt oxide (B) is at least 0.20g/cm³, and the difference in pressed density therebetween is at least0.1 g/cm³.

It is to be understood that although the present invention has beendescribed with regard to preferred embodiments thereof, various otherembodiments and variants may occur to those skilled in the art, whichare within the scope and spirit of the invention, and such otherembodiments and variants are intended to be covered by the followingclaims.

1. Lithium cobalt oxide comprising: a mixture of lithium cobalt oxideparticles (A) and lithium cobalt oxide particles (B); wherein thelithium cobalt oxide particles (A) are composed of uniformlymonodisperse particles, which refers to particles present independentlyof each other, and the lithium cobalt oxide particles (A) have anaverage particle diameter in a range of 5 to 30 μm; the lithium cobaltoxide particles (B) are composed of an aggregation of primary particlesforming a secondary particle, and the secondary particles have anaverage particle diameter in a range of 0.1 to 10 μm; and wherein a tapdensity of the mixture of the lithium cobalt oxide particles (A) and thelithium cobalt oxide particles (B) is at least 1.7 g/cm³, and a presseddensity of the mixture of the lithium cobalt oxide particles (A) and thelithium cobalt oxide particles (B) is 3.5 to 4.0 g/cm³.
 2. The lithiumcobalt oxide according to claim 1, wherein the tap density of themixture of the lithium cobalt oxide (A) and the lithium cobalt oxideparticles (B) is at least 2.0 g/cm³ and the pressed density of themixture of the lithium cobalt oxide (A) and the lithium cobalt oxideparticles (B) is 3.6 to 4.0 g/cm³.
 3. The lithium cobalt oxide accordingto claim 1, wherein the tap density of the mixture of the lithium cobaltoxide (A) and the lithium cobalt oxide particles (B) is 2.5 to 3.5 g/cm³and the pressed density of the mixture of the lithium cobalt oxide (A)and the lithium cobalt oxide particles (B) is 3.7 to 4.0 g/cm³.
 4. Thelithium cobalt oxide according to claim 1, wherein an average particlediameter of the lithium cobalt oxide particles (B) is smaller than anaverage particle diameter of the lithium cobalt oxide particles (A) inthe mixture of the lithium cobalt oxide particles (A) and the lithiumcobalt oxide particles (B).
 5. A nonaqueous electrolyte secondarybattery comprising a cathode plate having a cathode active material,wherein the cathode active material includes a mixture of lithium cobaltoxide particles (A) and lithium cobalt oxide particles (B); the lithiumcobalt oxide particles (A) are composed of uniformly monodisperseparticles, which refers to particles present independently of eachother, and the lithium cobalt oxide particles (A) have an averageparticle diameter in a range of 5 to 30 μm; the lithium cobalt oxideparticles (B) are composed of an aggregation of primary particlesforming a secondary particle, and the secondary particles have anaverage particle diameter in a range of 0.1 to 10 μm; and wherein a tapdensity of the mixture of the lithium cobalt oxide particles (A) and thelithium cobalt oxide particles (B) is at least 1.7 g/cm³, and a presseddensity of the mixture of the lithium cobalt oxide particles (A) and thelithium cobalt oxide particles (B) is 3.5 to 4.0 g/cm³.
 6. Thenonaqueous electrolyte secondary battery according to claim 5, wherein aratio of the mixture of the lithium cobalt oxide particles (A) and thelithium cobalt oxide particles (B) is (A):(B)=95:5 to 60:40 on a weightbasis.
 7. The nonaqueous electrolyte secondary battery according toclaim 5, wherein a ratio of the mixture of the lithium cobalt oxideparticles (A) and the lithium cobalt oxide particles (B) is(A):(B)=90:10 to 80:20 on a weight basis.
 8. The nonaqueous electrolytesecondary battery according to claim 5, wherein the average particlediameter of the lithium cobalt oxide particles (A) is within the rangeof 10 to 20 μm.
 9. The nonaqueous electrolyte secondary batteryaccording to claim 5, wherein the average particle diameter of thelithium cobalt oxide particles (B) is within the range of 2.0 to 8.0 μm.10. The nonaqueous electrolyte secondary battery according to claim 5,wherein the pressed density of the mixture of the lithium cobalt oxideparticles (A) and the lithium cobalt oxide particles (B) is 3.6 to 4.0g/cm³.
 11. The nonaqueous electrolyte secondary battery according toclaim 5, wherein the pressed density of the mixture of the lithiumcobalt oxide particles (A) and the lithium cobalt oxide particles (B) is3.7 to 4.0 g/cm³.
 12. The nonaqueous electrolyte secondary batteryaccording to claim 5, wherein the tap density of the mixture of thelithium cobalt oxide particles (A) and the lithium cobalt oxideparticles (B) is 2.0 to 3.0 g/cm³.
 13. The nonaqueous electrolytesecondary battery according to claim 5, wherein an average particlediameter of the lithium cobalt oxide particles (B) is smaller than anaverage particle diameter of the lithium cobalt oxide particles (A) inthe mixture of the lithium cobalt oxide particles (A) and the lithiumcobalt oxide particles (B).